The present invention relates to a high performance wafer-shaped semiconductor device having a tightly encasing plastic jacket. More specifically, it is an improvement over the semiconductor device disclosed in U.S. Pat. No. 3,986,201 granted to Herold et al on Oct. 12th, 1976.
In this prior art device, a wafer-shaped semiconductor element is provided with major electrodes on two major surfaces. A pair of heat dissipating cylindrical metal bodies are attached to respective ones of the major electrodes and an insulating ring encloses and centers the metal bodies and the semiconductor element. The insulating ring is held between two annular strips of sheet metal having inner edge regions that are fastened to the outer surfaces of the two metal bodies and outer edge regions that are embedded in a plastic jacket encasing the wafer-shaped semiconductor element, the jacket having been produced by casting or molding under pressure. However, it has been found very difficult to provide a plastic jacket housing as part of a wafer-shaped semiconductor device which will remain consistently tight over a broad temperature range of -40.degree. C. to +150.degree. C. and also after alternating thermal stresses. Alternating thermal stresses produce mechanical tensions in a device of the above-described type due to the different coefficients of thermal expansion of the metal bodies contacting the semiconductor element and the plastic used as a housing in the device. Such mechanical tensions lead to the formation of cracks in the plastic or cause the plastic to become detached from the metal strip embedded in the plastic. Reliable adhesion of the plastic to the metal parts is not assured, particularly if molding resins are used to encase the semiconductor element.
This occurs because molding resins, in order to be easily removed from the mold, contain additives in the form of mold release agents; i.e. substances which will facilitate removal from the mold. These mold release agents may prevent adhesion between the pressed plastic mass and the metallic housing portions to be encased thereby making it impossible to obtain a housing which is consistently tight.
It is an object of the present invention to provide a semiconductor device of the above-described type which permits the realization of a reliable bond between the metallic housing portions and the pressed plastic mass so that the resulting housing portecting the semiconductor element is pressure-tight as well as oil-tight.